The present invention relates to an apparatus and method for providing soft switching of parallel switch assemblies, and more particularly to soft switching of parallel switch assemblies of a switch mode power converter.
Conventional switch mode power converters employ controllable switch assemblies which each include a switch to alternately connect an inductor between the input and the output of the converter. Diodes are included in the switch assembly to provide isolation between the converter power input and output when the switch is closed.
As is well known in the art, the switches of such switch mode power converters lose substantial amounts of power during switching events. Regardless of the application of these power converters, for example, a motor speed control, a gradient amplifier, or a power supply, high power may exist across the switch terminals at the time the switch is activated and shortly thereafter. The diodes in the circuit typically will not cease conducting immediately when the switch is activated, but will allow reverse current to flow temporarily before xe2x80x9crecoveringxe2x80x9d the ability to block reverse current and maintain reverse voltage.
The difficulty of constructing and implementing power converter switches increases as the current demands on the switches increase. Very large semiconductors, for example, often have low initial yields, generate significant heat and require special thermal mounting, and are difficult to electrically connect to other components in the circuit. Thus, most high current switches are made from parallel assemblies of smaller switches which are easier to construct. One tradeoff associated with the use of parallel switch assemblies, however, is the need to ensure accurate current sharing among the switches. Clearly, depending upon the electrical characteristics of each switch, there could be situations where one switch turns on before the other switches connected in parallel, or turns off, after the other switches. If one or more switches continues to operate at times different from the remaining switches, that switch may experience greater power dissipation and, over time, may fail.
Some conventional switch mode power converters employ an auxiliary power switch to achieve the enhanced efficiency of zero voltage switching (xe2x80x9cZVSxe2x80x9d) wherein the auxiliary switch is operated first to permit the voltage across the main switch to drop to zero before the switch is turned on. The auxiliary switch is typically coupled through a small inductor to the main switch circuit at the junction between the main switch and the anode of the switch assembly diode. Since the diode would generally be conducting such that it must be xe2x80x9creverse recoveredxe2x80x9d before the main switch can approach zero volts, the small inductance of the auxiliary switch can recover, rather than dissipate, the energy of the stored charge of the diode and the main switch circuit. Such auxiliary switches are generally configured to return the recovered energy to the main switch circuit after the ZVS event. However, additional switches result in additional expense which may prohibit the use of auxiliary switches in certain applications.
The present invention provides an apparatus and method for soft switching of parallel switch assemblies in a switch mode power converter including a plurality of switch assemblies joined by small inductances to a common node and individually controlled by a switch controller. The inductances fimction as resonant auxiliary switch inductors as well as traditional fast current sharing impedances. The switch controller enables a subset of switch assemblies (typically, one) during each switching cycle before enabling the remaining switch assemblies. The first enabled switch assembly hard recovers the energy stored in its diode, and soft recovers the diodes of the remaining switch assemblies. Additionally, the switch controller may disable a second subset of switch assemblies (typically, one) after the remaining switch assemblies have been disabled during each switching cycle. This zero current turn-off (xe2x80x9cZCTxe2x80x9d) technique ensures that most of the turn-off power loss occurs at the switch assembly which is disabled last. Moreover, the switch controller commutes the switch assemblies selected for the first and second subsets for each switching cycle.
The switch controller includes a clock circuit which provides a clock signal to a pulse width modulator. The pulse width modulator also receives a demand signal, such as the current demand of the power converter load, and outputs a modulated version of the demand signal. By passing the modulated signal through one or more delay circuits, and logically relating the delay signal(s) to the clock signal and/or the modulated signal, the switch controller provides enable inputs to the switches of each of the switch assemblies in a sequence that commutes the selection of the early enabled and late disabled switch assemblies. By selecting a different switch to be the first enabled and last disabled each switching cycle, the switch controller more evenly distributes the power losses and the attendant stresses across all of the switch assemblies included in the power converter.